Electronic apparatus

ABSTRACT

A hollow cathode lamp is controlled during normal running, by a pulsing circuit, and the lamp is in series with a diode which protects the pulsing circuit when the lamp is initially ionised by a large voltage applied across the anode and cathode of the lamp. The pulsing circuit drives an operational amplifier which in turn drives the control terminal of a transistor in series with the lamp. The operational amplifier presents a high impedance to the pulsing circuit, and therefore a number of similar lamps can be controlled, each through its own amplifier, without interaction.

United States Patent 1191 1111 3,91 1,319

Brown 51 Oct. 7, 1975 [5 ELECTRONIC APPARATUS 3,309,567 3/1967 Flieder et al 315/176 3,449,630 6/1969 Roth et al. 315/175 X Inventor: Edward Bmwh Chahock 3,497,768 2/1970 Mathisen 315 171 x near Ashford, England [73] Assignee: The Rank Organisation Limited, P i E i N h K f London, England Attorney, Agent, or Firm-Brisebois & Kruger [22] Filed: May 17, 1974 [21] App]. No.: 471,045 ABSTRACT A hollow cathode lamp is controlled during normal [30] Fo i Application P i it D t running, by a pulsing circuit, and the lamp is in series May 17 1973 United Kingdom 23602/73 with a diode which Protects the pulsing chchh wheh the lamp is initially ionised by a large voltage applied 52 11.8. C1. 315/176- 315/171- 315/174 moss the anode and cathcde the The pulsing 51 lm. c1. .3 51; 41/00 circuit hives an operational amplifier which in mm [58] Field of Search 315/ 171 174 175 drives the control terminal ofa transistor in series with 3 the lamp. The operational amplifier presents a high impedance to the pulsing circuit, and therefore a num- [56] References Cited ber of similar lamps can be controlled, each through UNITED STATES PATENTS its own amplifier, without interaction.

3,238,415 3/1966 Turner 315/176 X 4 Claims, 1 Drawing Figure 1 L I I '1 L i -)'1 K5 ,4? 1 6 i 1y i V A L l 1' i /i P 1 I 1 171.33 P/ l l 1 l 1 1 ,1---- 2 i l W I? l I LI, l 1. QLJ J J. L kl I 1 J US. Patent Oct. 7,1975

ELECTRONIC APPARATUS The present invention relates to an electrical driving circuit for hollow cathode lamps such as are used in atomic absorption spectroscopy.

The technique of atomic absorption spectroscopy is now well established, and in such technique radiation emitted from a hollow cathode lamp is caused to pass through a flame into which is passed the material, usually a solution to be analysed. The absorption of the particular resonance line is so measured to give the concentration of the particular substance in the material to be identified.

The effective electrical driving of the hollow cathode lamp is an important consideration in this technique and this invention, at least in its preferred form, is concerned with providing a low cost, high efficiency driving circuit for a hollow cathode lamp. The invention furthermore can be applied in any technique requiring the driving of a hollow cathode lamp in a manner similar to that in which it is driven in atomic absorption spectroscopy.

Conventionally, hollow cathode lamps are driven in one of three methods as follows:

1. D.C. energised 2. D.C. energised and pulse modulated 3. D.C. energised in combination with mechanical- /optical beam chopper means.

This invention is concerned with D.C. energised and pulse modulated operation.

In conventional driving circuits for hollow cathode lamps, the ionising voltage applied to the lamp, which is normally higher than the running voltage, is applied across the modulating or pulsing components of the driving circuit, requiring such components to be more robust, and therefore more expensive than they really need be.

It is desirable in these driving circuits to include current control means whereby the lamp current can be varied. conventional control means have tended to be non-linear in their operation and this has led to the necessary requirement of providing a current meter either directly in series with the lamp or each lamp or switchable into series with the or each lamp to monitor the current thereof. As high voltages are normally involved in these circuits, accordingly, limitations are placed on the meter or meters and the wiring thereof.

Accordingly, the objects of two preferred aspects of the present invention, are to obviate or mitigate these disadvantages.

According to a first aspect of the invention there is provided an electrical driving circuit for a hollow cathode lamp comprising a diode means of which one terminal is connected to the cathode of the lamp, and D.C. pulsing means is connected to the other terminal so that current flow through the D.C. pulsing means is prevented by such diode means upon application of a high striking voltage directly between the anode and cathode of the lamp with a lower running voltage applid to the anode and across the circuit comprising lamp, diode means and D.C. pulsing means.

The circuit preferably has a ground line between which and the anode of the lamp, the running voltage is in use applied.

The D.C. pulsing means preferably includes a square wave generator to the output which is connected to a potentiometer, the other terminal of which is connected 'to'fthe ground line and the potentiometer pointer being connected to one input terminal of a two input terminal operational amplifier, the other terminal of which being connected between a series regulating transistor and a balancing resistor in the series circuit including lamp, diode means, series regulating transistor, balancing resistor and ground line, the output of said operational amplifier being connected to the control terminal of the series regulating transistor.

Preferably, there is a light emitting diode connected in the series circuit between the series regulating transistor and the diode means.

The operational amplifier preferably presents a high input impedance to D.C. pulsed modulation supplied thereto from the potentiometer pointer, so that other lamps having identical driving circuits, apart from the square wave generator, can be driven from said square wave with little or no interaction.

With this driving circuit, a linear change in potentiometer setting produces a linear change in lamp current, eliminating the need for meter switching arrangements and expensive meters. Indeed, by using a light emitting diode, the lamp current is constantly monitored because not only does the light emitting diode indicate the on-off mode of the lamp, its radiation intensity varies linearly with lamp current and it will therefore visually indicate the current level.

In a multiple lamp arrangement, each lamp will have a driving circuit as aforesaid, except for the square wave generator, and a single square wave generator is provided having its output connected to each of the potentiometers.

The invention also provides an atomic absorption spectrophotometer having a hollow cathode lamp to which is connected a driving circuit as aforesaid.

Preferably, the photometer will have a plurality of said hollow cathode lamps each having a driving circuit as aforesaid, except for the square wave generator, and a single square wave generator is provided having its output connected to each of the potentiometers.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing of which the single FIGURE shows a circuit includingg a hollow cathode lamp and a driving circuit according to the invention connected thereto.

Referring to the drawing the lamp is indicated by reference L and it will be seen that the anode of the lamp is connected to a positive line 10. The cathode of the lamp L is connected to a diode means in the form of a diode switch D1 whose other terminal is connected to a light emitting diode LED. The lamp L, diode D1 and light emitting diode LED are connected in a series circuit which further includes a series regulating transistor T and a balancing resistor R, such resistor R finally being connected to a ground line 12.

The circuit further includes D.C. pulse modulating means in the form of circuitry including a square wave generator G, a potentiometer P and an operational amplifier A. The output of the square wave generator G is connected to one terminal of the potentiometer P and the other terminal of the potentiometer is connected to the ground line 12. The potentiometer has a wiper pointer Pl which is connected to one input of the operational amplifier A and a second input of the operational amplifier A is connected to the aforesaid series circuit as shown between transistor T and balancing resistor R. In operation of the circuit illustrated, initially a large DC. voltage VS is applied directly between cathode and anode of the lamp L to ionise the gas contained therein. In this ionisation step, diode D1 acts to prevent any current flow through the remainder of the aforesaid series circuit. When the voltage at the cathode of lamp L goes positive relative to ground line 12, i.e., when VS, the strike voltage, is less than VR, the running voltage, diode D1 allows current to flow through the series circuit comprising resistor R, transistor T, light emitting diode LED and diode D1.

The value of the current flowing through the lamp L is dictated by the voltage V2 between pointer P1 and ground line 12 and at steady operational conditions the lamp current flowing in resistance R develops across this resistance a voltage V1 which is substantially equal to the voltage V2. In practice, since the gain of the operational amplifier A is not infinite, a small voltage difference will exist between V1 and V2 which identifies the state of conductance of the series regulating transistor T which in turn sets the current level. Hence, as the current through the lamp L tries to change due, for example, to the change in the temperature of the lamp or in the value of the applied voltage VR, then a change in V1 occurs which causes the amplifier output voltage to be driven in such a direction as to cause a change in the conductance of the series regulator transistor, which reduces the change in current to a negligible proportion. The circuit therefore, it will be seen, is selfcompensating and will endeavour to maintain a constant current through the lamp L which of course is desirable for hollow cathode lamps used in atomic absorption spectroscopy.

The reference wave form from the square wave generator G is injected into the non-inverting input of the operational amplifier. The input impedance presented by the amplifier to this reference wave form is sufficiently high to present negligible loading on the current setting potentiometer P and therefore a number of lamps, one of which is shown in dotted lines in the figure, can be driven from the same reference source, namely square wave generator G, with negligible interaction between circuits. A linear change in the potentiometer setting produces a linear change in the V2 which in turn produces a linear change in lamp current.

The adoption of the light emitting diode in series with the lamp is a useful monitoring means for indicating the condition of operation of the lamp. Firstly, it will clearly give the on-off indication of the lamp but furthermore, as its radiation intensity varies linearly with the current therethrough, it also gives an indication of lamp current.

The embodiment described presents a number of advantages over prior arrangements. In particular, the high ionising voltage is applied only across the lamp and not across the modulating circuitry and therefore this ensures maximum efficiency and cost effectiveness of the parts of the circuit that are used for the stabilising and modulation of the lamp current.

The achievement of the linear relationship between adjustment of the potentiometer and lamp current eliminates the need for current setting meters or switch/combinations which increase costs and make operation more difficult.

The use of a series regulating transistor for control and regulation of the lamp current presents an advantage over valve or transformer regulating means which are bulkier and less efficient than transistors.

The driving of additional lamp circuits from a single modulating square wave generator simplifies subsequent circuits.

A circuit as illustrated in the drawing is of high efficiency and from the use of solid state devices is compact, and because of ease of manufacture and simpler instrument design it is relatively cheap to produce.

I claim:

I. Hollow cathode lamp circuit means comprising a hollow cathode lamp having an anode and a cathode and a driving circuit for the lamp, said driving circuit comprising a. diode means having first and second terminals;

b. means connecting the first terminal of the diode means to the cathode of the lamp;

c. d.c. pulsing means;

d. means connecting the dc. pulsing means to the second terminal of the diode means;

e. means enabling the application of a high striking voltage across the lamp only; and

f. means including a ground line enabling the application of a running voltage, which is lower than the striking voltage, which is across the series circuit comprising the lamp, diode means, and dc. pulsing means; said d.c. pulsing means including:

a square wave generator having an output;

a potentiometer having first and second terminals and a pointer output;

i. means connecting the output of the square wave generator to the first terminal of the potentiometer;

j. means connecting the second potentiometer terminal to the ground line;

k. an operational amplifier having first and second outputs and an output;

1. means connecting the potentiometer pointer output to the first input of the operational amplifier;

m. a series regulating transistor having two conducting terminals and a control terminal;

n. means connecting the first conducting terminal of the regulators transistor to the second terminal of the diode means;

0. a balancing resistor having first and second terminals;

p. means connecting the first balancing resistor terminal to the second conducting terminal of the regulating transistor;

q. means connecting the second balancing resistor terminal to the ground line;

r. means connecting the junction between the balancing resistor and regulating transistor to the second input of the amplifier; and

s. means connecting the output of the amplifier to the control terminal of the regulating transistor.

2. Hollow cathode circuit means in accordance with claim 1 including a light emitting diode forming the means connecting the first terminal of the diode means to the cathode of the lamp.

3. Hollow cathode lamp circuit means in accordance with claim 1, wherein the operational amplifier has a high input impedance.

4. Hollow cathode lamp circuit means comprising a plurality of hollow cathode lamps and having an anode and a cathode, means for applying a striking voltage across the anode and cathode of the lamps, and for applying a lower running voltage across the lamps, a common pulsing generator for driving all of the hollow cathode lamps a translation circuit for each lamp, circuit means connecting the pulsing generator to the associated lamp, each translating circuit comprising:

a. diode means having first and second terminals; b. means connecting the first terminal of the diode means to the cathode of the lamp; c. a ground line; d. a potentiometer having first and second terminals and a pointer output; e. means connecting the output of the square wave generator to the first terminal of the potentiometer; f. means connecting the second potentiometer terminal to the ground line; g. means an operational amplifier having first and second inputs and an output; h. means connecting the potentiometer pointer output to the first input of the operational amplifier;

i. a series regulating transistor having two conducting terminals and a control terminal;

j. means connecting the first conducting terminal of the regulators transistor to the second terminal of the diode means.

k. a balancing resistor having first and second terminals;

1. means connecting the first balancing resistor tenninal to the second conducting terminal of the regulating transistor;

m. means connecting the second balancing resistor terminal to the ground line,

n. means connecting the junction between the balancing resistor and regulating transistor to the second input of the amplifier; and

0. means connecting the output of the amplifier to the control terminal of the regulating transistor. 

1. Hollow cathode lamp circuit means comprising a hollow cathode lamp having an anode and a cathode and a driving circuit for the lamp, said driving circuit comprising a. diode means having first and second terminals; b. means connecting the first terminal of the diode means to the cathode of the lamp; c. d.c. pulsing means; d. means connecting the d.c. pulsing means to the second terminal of the diode means; e. means enabling the application of a high striking voltage across the lamp only; and f. means including a ground line enabling the application of a running voltage, which is lower than the striking voltage, which is across the series circuit comprising the lamp, diode means, and d.c. pulsing means; said d.c. pulsing means including: g. a square wave generator having an output; h. a potentiometer having first and second terminals and a pointer output; i. means connecting the output of the square wave generator to the first terminal of the potentiometer; j. means connecting the second potentiometer terminal to the ground line; k. an operational amplifier having first and second outputs and an output; l. means connecting the potentiometer pointer output to the first input of the operational amplifier; m. a series regulating transistor having two conducting terminals and a control terminal; n. means connecting the first conducting terminal of the regulators transistor to the second terminal of the diode means; o. a balancing resistor having first and second terminals; p. means connecting the first balancing resistor terminal to the second conducting terminal of the regulating transistor; q. means connecting the second balancing resistor terminal to the ground line; r. means connecting the junction between the balancing resistor and regulating transistor to the second input of the amplifier; and s. means connecting the output of the amplifier to the control terminal of the regulating transistor.
 2. Hollow cathode circuit means in accordance with claim 1 including a light emitting diode forming the means connecting the first terminal of the diode means to the cathode of the lamp.
 3. Hollow cathode lamp circuit means in accordance with claim 1, wherein the operational amplifier has a high input impedance.
 4. Hollow cathode lamp circuit means comprising a plurality of hollow cathode lamps and having an anode and a cathode, means for applying a striking voltage across the anode and cathode of the lamps, and for applying a lower running voltage across the lamps, a common pulsing generator for driving all of the hollow cathode lamps a translation circuit for each lamp, circuit means connecting the pulsing generator to the associated lamp, each translating circuit comprising: a. diode means having first and second terminals; b. means connecting the first terminal of the diode means to the cathode of the lamp; c. a ground line; d. a potentiometer having first and second terminals and a pointer output; e. means connecting the output of the square wave generator to the first terminal of the potentiometer; f. means connecting the second potentiometer terminal to the ground line; g. means an operational amplifier having first and second inputs and an output; h. means connecting the potentiometer pointer output to the first input of the operational amplifier; i. a series regulating transistor having two conducting terminals and a control terminal; j. means connecting the first conducting terminal of the regulators transistor to the second terminal of the diode means. k. a balancing resistor having first and second terminals; l. means connecting the first balancing resistor terminal to the second conducting terminal of the regulating transistor; m. means connecting the second balancing resistor terminal to the ground line, n. means connecting the junction between the balancing resistor and regulating transistor to the second input of the amplifier; and o. means connecting the output of the amplifier to the control terminal of the regulating transistor. 